In an optical communication system, it is important to detect a trouble such as a damage of an optical fiber line or an increase in the transmission loss. Particularly, in a subscriber optical communication system, the point of the trouble must quickly be detected and restored when a trouble such as breakage or increase of the transmission loss occurs in an optical fiber line or optical network unit. The use of so-called Passive Optical Network (PON) system has recently been spreading in the subscriber optical communication system. In the PON system, the connection between a central office and subscribers are made using a first optical fiber line extending from the central office, optical splitters, and a plurality of branched second optical fiber lines. This helps to decrease the cost incurred per subscriber with respect to the first optical fiber line and optical line terminals installed in the central office.
To detect the above-mentioned troubles in an optical communication system, a remote fiber test system is generally provided. A known remote fiber test system is, for example, the system described in Y. Enomoto et al.: J. Optical Networking, vol. 6 (2007) 408 (Non-patent literature 1). This remote fiber test system consists of optical fiber line test equipment, a reflection filter provided in an optical fiber line, and information on installation such as the position of installed reflection filters, etc.
The optical fiber line test equipment, which utilizes a reflectometry technique, detects the position of a fault on the basis of features such as peaks and level differences in the reflectance distribution of an object under measurement such as an optical fiber line. (In this specification, the “reflected light” means Fresnel reflected light and Rayleigh scattered light, unless otherwise specified in particular.) In the PON system, it is necessary to receive reflected light from a plurality of second optical fiber lines at the same time and to detect the respective features of the reflectance distributions by distinguishing each second optical fiber line. Therefore, it is required that the optical fiber line test equipment be capable of measuring reflectance distributions with high spatial resolution.
A known reflectometry technique is Optical Time Domain Reflectometry (OTDR) for measuring reflectance distributions based on temporal variation in the intensity of reflected light that occurs when pulsed probe light propagates through an object under measurement. To obtain a high spatial resolution with the OTDR, it is necessary to make the pulse width of the probe light to be narrow. Also, it is necessary to increase the power of the probe light so as to compensate for the decrease in the signal to noise ratio (SNR) due to decrease in the energy of the probe light. However, if the power of the probe light is increased, the degradation of measurement performance and the interference in the communication signal will occur because of a nonlinear optical phenomenon such as stimulated Brillouin scattering in the object under measurement. Therefore, in the OTDR, the spatial resolution is limited to about several meters.
Another known reflectometry technique is Optical Coherence Domain Reflectometry (OCDR) (For example, K. Hotate and Z. He: J. Lightw. Technol., vol. 24 (2006) 2541 (Non-patent literature 2), T. Saida and K. Hotate: IEEE Photon. Technol. Lett., vol. 10 (1998) 573 (Non-patent literature 3), Z. He and K. Hotate: J. Lightw. Technol., vol. 20 (2002) 1715 (Non-patent literature 4)). In the OCDR, the reflectance at a specific position in an object under measurement is measured by utilizing the principle that the magnitude of interference between reflected light, which occurs when probe light having a comb-shaped optical coherence function due to modulation of light frequency travels through the object under measurement, and reference light, which is a branched part of the probe light, depends on difference in the delay time between the reflected light and the reference light. Moreover, in the OCDR, the reflectance distribution of the object under measurement is sought, changing the position of reflectance measurement by altering the interval of the light frequency modulation in the probe light, or the like. With the OCDR, a higher spatial resolution can be obtained as compared with the OTDR. In Non-patent literature 2, for example, it is shown that the reflection point located at a distance of 5 km can be measured with a spatial resolution of 19 cm.
The optical coherence function is such that the autocorrelation function V(t)V*(t−τ) of an electric field V(t) of the light that is a function of time t as a variable is normalized with the light intensity and also the Fourier transform of optical power spectrum normalized with the light intensity. When light of electric field V(t) is split into two and the delay time difference between these two split light is τ, the magnitude of the interference fringe of these two split light is represented by the real part of the optical coherence function of the light. Also, the absolute value of the optical coherence function is called a degree of coherence, and shows the magnitude of the interference.
Probe light used in the OCDR is, for example, light in which the light frequency is modulated at constant time intervals in a manner such as in the order of f0, f0+fs, f0−fs, f0+2 fs, f0−2 fs, f0+3 fs, f0−3 fs, . . . , or light in which the light frequency is modulated with the modulation frequency fs in a form of sine wave. The optical coherence function of the probe light in which the light frequency is modulated in such manner has peaks (coherence peaks) having a shape similar to a delta function when fsτ is an integer. That is, these probe light have a comb-shaped optical coherence function. When fs changes, the position of coherence peaks also changes.
The comb-shaped optical coherence function has a plurality of coherence peaks arranged at an interval 1/fs. By means of restriction made by a gate with a time width that is shorter than the interval 1/fs of arrangement of the coherence peaks, a pulse of the probe light is cut out so that one of the coherence peaks may exist in the measurement section of the object under measurement.
Non-patent literature 2 describes a technique with which the distance range that can be measured according to the OCDR is expanded. In the technique, light having an optical coherence function including a number of comb-like coherence peaks is generated by periodical frequency modulation of a light source, and by pulsing the generated light, an optical coherence function having a single coherence peak is made. Moreover, the delay time is made coincident with the coherence time of the light source either by altering the delay time of the reference light by switching the delay fiber that is provided in the reference optical path, or by causing the reference light to propagate through a loop circuit including a delay line. In such manner, it is made possible to measure a long distance range on the order of km.
Also, Non-patent literature 3 refers to a technique for expanding the distance range that can be measured with the OCDR. In this technique, the measurement distance range is expanded by choosing a light frequency modulation period so that the coherence peak of the optical coherence function may exist in a distance range that exceeds the coherence length of the output light of a light source. It is mentioned that by way of example the reflected light occurring at a distant point of about 5 km can be measured by forming coherence peaks at about 1 km intervals by a frequency modulation performed at a modulation period of 100 kHz±10 kHz for the light source having a coherence length of 60 m.